cmd/api: don't print out except.txt removed features to stdout

It's just noise. They've already been acknowledged in except.txt.

R=golang-dev, r
CC=golang-dev
https://golang.org/cl/9392047

src/cmd/api/goapi.go

@@ -231,7 +231,12 @@ func compareAPI(w io.Writer, features, required, optional, exception []string) (
 		case len(features) == 0 || (len(required) > 0 && required[0] < features[0]):
 			feature := take(&required)
 			if exceptionSet[feature] {
-				fmt.Fprintf(w, "~%s\n", feature)
+				// An "unfortunate" case: the feature was once
+				// included in the API (e.g. go1.txt), but was
+				// subsequently removed. These are already
+				// acknowledged by being in the file
+				// "api/except.txt". No need to print them out
+				// here.
 			} else if featureSet[featureWithoutContext(feature)] {
 				// okay.
 			} else {

src/cmd/api/goapi_test.go

@@ -110,7 +110,7 @@ func TestCompareAPI(t *testing.T) {
 			features:  []string{"A", "C"},
 			exception: []string{"B"},
 			ok:        true,
-			out:       "~B\n",
+			out:       "",
 		},
 		{
 			// http://golang.org/issue/4303